The open microwave resonator technique with a measuring hole in one of the mirrors was developed for investigation of electromagnetic properties of conductors smaller than 5 mm. It was employed for measurement of the electric and magnetic polarizabilities of the aerogels based on multiwalled carbon nanotubes (MWCNT). MWCNT aerogels showed electric polarizability intermediate with that of the metal conductors and dielectrics with values close to metals within the frequency range studied (8–12 GHz). This characteristic in combination with the lower weight of MWCNT aerogels and rigid structure makes them promising material for such industrial fields as plasmon resonance and acoustic shielding.Spherical MWCNT aerogels with different diameter.

Lightweight materials for efficient electromagnetic interference shielding over a wide range of frequency are of great interest. Aerogels of multi-walled carbon nanotubes (MWCNTs) distributed in a dielectric matrix may interact effectively with electromagnetic radiation. Moreover, being arranged in specific patterns, they can also provide metamaterial characteristics (e.g., negative refractive index). In the present paper, we have studied the interaction between two spheres of the carbon nanotube aerogels in order to obtain a comprehensive basis for producing periodic 2D and 3D structures of such aerogel spheres in a dielectric matrix. We have extended the open resonator technique to the case of two units of MWCNT aerogels in order to reveal the interaction between them. The performance of the system depends on the orientation of the electric field to the axis of the aerogel cluster: MWCNT aerogel spheres (d ¼ 4.5 mm) interact considerably at the gap between spheres $0.2λ (11.6 GHz) for perpendicular orientation, while, in case of axial orientation, the interaction is negligible even at the distances close to the contact between aerogels ($0.02λ). Additionally, the cluster of two carbon nanotube aerogel spheres provides the polarizability close to that of copper spheres while having only 1% of their weight.

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Here a method for estimating the effective parameters (effective conductivity) of spherical aerogels of multilayer carbon nanotubes (CNTs) is proposed using the measured values of their polarizability and the wire medium model. While aerogels have a structure formed with tangles of curved nanotubes with chaotic contact distribution, the possibility to describe their electrophysical properties using the regular connected wire medium is estimated here. Calculations of the polarizability of regular structures consisting of copper and CNT wires of identical diameters (not less than 50 nm) for different models are performed. It is suggested that by selecting the parameters of a regular medium, it is possible to achieve the closeness of the polarizability values of samples of the same shape consisting of regular and irregular media. The calculated polarizability values of aerogels are consistent with the results of measurements of polarizability performed using the method of open resonators in the range 8–12 GHz. The possibility of estimating the electrophysical characteristics of carbon aerogels is considered for further modeling of the electromagnetic characteristics of composites based on them.

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Results of calculations of polarizability of a metal rod by various approximation methods, numerical solution of an integral equation, and polarizability measurement by the resonator method are presented. Methods of calculations of the wave resistance of a symmetric vibrator are examined, and a method of estimating the wave resistance for small ratios of the vibrator length to its diameter is suggested.